1. Field of the Invention
The present invention relates generally to the construction of load bearing structures to resist destruction during earthquakes, and more particularly, it relates to a support system able to minimize seismic effects.
2. Description of the Prior Art
Throughout the world and especially along the Pacific Coast of the Continental United States of America, including Mexico, and in many sections of the Southern Hemisphere, including the South American countries, earthquakes are very common occurrences. In fact, in the islands of the Pacific Ocean, many of these seismic disturbances occur at regular intervals. The strong seismic disturbances, or earthquakes, frequently result in a substantial loss of human lives and building failures.
Earthquakes comprise horizontal and vertical ground shocks or seismic vibrations. During an earthquake, structures such as buildings and the like, which are connected to the earth by conventional foundations, are subjected to forced vibrations due to the induced forces in their foundations. The inertia of the building structure tends to greatly resist such earthquake-induced movements transmitted from the earth to the foundation of the building. As a consequence, a lateral shearing force, that is, base shear, is applied to the structural body at its foundation. The magnitude of this base shearing force is clearly a major factor in earthquake damage and has long been the principal problem faced by the structural engineer in designing a building. Such forces to which a structural body are subjected during such seismic disturbances are, based on specific dynamic characteristics of the structure and for all practical purposes, directly portional to the weight (mass) of the structural body and, therefore, the deleterious effects of an earthquake can be minimized to some degree by the use of light-in-weight construction materials and by designing structures of relatively low total weight. However, the structural design engineer is constrained in this approach because of both the limited choices and relatively high cost of current commercially-available materials for use in this application.
Further, in some structures, such as a nuclear power facility, damage caused by earthquakes constitutes a special safety problem because of the possibility that nuclear materials may be released. Beyond the well-recognized understanding that such nuclear materials are radioactive and such radioactivity poses a significant problem to life, in general, it is little-known, high toxicity and poisonous characteristics of nuclear materials. For these, and other reasons, such a structure must have a substantially higher safety factor when it is built to eliminate the likelihood of exposing the surrounding population to toxic products and excessive radiation.
Structural failures in buildings, and the like, occur when extreme earthquake forces and earth displacements are transmitted to columns or posts via footings.
One reason for such failure lies in the inability of connections between columns and footings to sufficiently isolate the building from lateral movement of earth, that is, movement which is generally transverse to the column height direction.
In the prior art, there exists several ways to build buildings which increase the probability of the building surviving an earthquake. Such structures typically address the problem by attempting to provide a building structure which will survive such a seismic attack.
One of the structural design approaches used in the prior art was to utilize currently available commercial materials and to simply increase the size of all structural members so that it could withstand an earthquake of a certain prescribed seismic intensity. However, earthquakes do not have a predetermined approach to its effect upon the building or structure. The typical rapidly reversing forces inherent in the earthquake create such an effect upon the structure that it effects internal and external portions of the building and its structure. On the other hand, the present prevailing approach depends on elasto-plastic behavior of building structure to simply prevent the collapse of the building. However, this approach allows structural and nonstructural damages and permanent deformations, which may be acceptable as far as the building code goes, but may be unacceptable from a practical use/aesthetic standpoint.
As can be readily observed, however, the basic problem is how to isolate the building or structure from the devastating effects of strong earthquakes, since simply increasing the structural strength of the building, or depending on plastic deformation of the building structure, would anticipatorily fail as a satisfactory solution to the problem.
Those prior art devices known to the inventor herein are United States Patent Number:
U.S. Pat. No. 2,055,000 PA1 U.S. Pat. No. 3,350,821 PA1 U.S. Pat. No. 3,730,463 PA1 U.S. Pat. No. 3,762,114 PA1 U.S. Pat. No. 4,187,573 PA1 U.S. Pat. No. 4,222,206 PA1 U.S. Pat. No. 4,328,648 PA1 U.S. Pat. No. 4,330,103
However, none of the above-identified patents are considered to be similar to the present invention disclosed herein with the possible exception of U.S. Pat. No. 4,328,648.
U.S. Pat. No. 4,328,648 relates to means for protecting structures from earthquakes, explosions, cyclones and other sources of sudden external shock, said means including (as shown in FIG. 1), a support system comprised of a base 28, a pedestal 39, hanger rods 44, and an elastomer element 36. The fundamental purpose of the means disclosed in this patent is to isolate a building from the effects of seismic shock which may be transmitted through the building's foundation to the structure forming the building. One of the many problems posed by this invention and its application for the purposes intended relates to the fact that, as shown in FIG. 4 of the Drawings, the open side of the socket 47 where a hanger rod 44, which is threadably coupled with the ball bearing 43 via the threaded engagement indicated at 46 is captively secured within the socket formed in the base 47. When movement occurs, the operation of this system may possibly be impaired due to overstress near the edge. The overstressing referred to herein specifically relates to the overstressing which would occur due to the stress concentration found at the point of contact between the ball bearing 43 and the socket edge (most nearly identified by number 48 in FIG. 4). Further, such stress concentration would also exist in the reduced section around the edge 48. The failure of the support system means the destruction of the building. Additionally, it should be clearly noted that the shock insulation means disclosed in this patent possesses no inherent movement dampening effect, and attempts to isolate a building or structure from seismic shock effects by simply depending on the pendulous motion inherent in this structure, and, as a consequence, there exists no means for controlling highly deleterious and unacceptable harmonic vibration components. In fact, such harmonic vibration components can result in the damage of nonstructural items in the building and cause human discomfort.
U.S. Pat. No. 4,187,573 also teaches and discloses the use of an elastomer pad as an inherent, and, substantially essential part of the seismic isolation means disclosed in the patent.
In fact, it is well-known in the prior technical literature that various schemes have been attempted which employ the soft first story concept as a means by which to permit lateral movement of the support structure subjected to seismic activity and to absorb some of the energy produced thereby. However, the use of the soft first story concept has largely been demonstrated to be unfeasible for the reasons summarized in a technical article written by K. Staudacher in the "Proceedings of the Structural Engineers Association of Southern California," 1983, entitled "The Swiss Full Base Isolation System (3D) for Extreme Earthquake Safety of Structures." The article emphasizes the use of rubber bearings as building isolators rather than the soft first story concept.
Basically, the use of such elastomer pads, such as rubber, as base isolators raises significant questions about public safety. Buildings are extremely heavy in weight. In using such elastomer pads underneath the supporting columns and on top of the foundation, the building is, in fact, being supported on the elestomer pads. Should the pads malfunction, or in the event of an unexpectedly strong earthquake, such could practically postulate the failure of the elastomer isolator. Following such failure, the building will drop from a height equal to the thickness of the elastomer pad. The anticipated net effect of such an impact would result in the destruction of the building and likely lead to the total collapse of the building.
It is for these problems, and reasons, as well as the many others which are clearly set forth hereinafterwards that the instant invention was developed to solve.